In a large number of measuring devices, either the exciter signals or the measurement signals, from which information concerning the process variables are derived, are digitized, in order, therewith, to simplify and improve the signal processing. Such is described, for example, in published international application, WO 2004/102133 for the case of a capacitive, fill-level, measuring device.
The signals to be processes are, most often, electrical signals, which are either alternating voltages or are transmitted in alternating voltages. These alternating voltages are then converted into digital signals by an analog-digital converter. In such case, the analog signals are sampled at sampling points, which are usually spaced equally in time. The resolution and, therewith, the accuracy, possessed by the digital signal, depends, in such case, on the number of sampling points. Due to available memory, available energy or achievable sampling rate of the analog-digital converter, on occasion, the resolution cannot be increased as much as desired. In order, in spite of such constraints, to increase the resolution, there is the so-called equivalence-time method. This can be applied, especially, to periodic signals. In such method, a signal is either multiply produced or multiply sampled with a fixed sampling rate. At each sampling, the time points of the sampling are offset, or, since the sampling rate is fixed, the point in time for the first sampling point is offset. At each sampling, thus, the same number of points are sampled. Since these, are, however, between the individual samplings, offset relative to one another, correspondingly many data sets result, which, together, describe the entire signal, wherein the digitized data must be appropriately processed. See, in this connection, German Patent DE 4434688 A1, published international application, WO 03/019120 A1, or Japanese patent, JP 05041094A. This method is usually accomplished in such a manner, that the sampling is appropriately offset in the analog-digital converter.